| Warning |
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| Your mileage will vary depending on the details of your system! |
In this experiment, we study the "batch optimization", that maximizes RAM bandwidth usage, by issuing a lot of load commands all at once (for a batch), then letting the processor run a "payload" of other instructions. Essentially, since all the loads from main memory are done in parallel, it is faster than issuing a load, processing an instruction, issuing another load, processing another instruction...
We randomly access values in a big, statically allocated array (that doesn't fit in L3), and perform various amounts of processing on those values. "Batching" consists in grouping the lookup and processing of values into "batches" of small size. We want to understand what is the optimal batch size, and how the "batching optimization" depends on the amount of work done on each value.
The setup that produced the results in the folder linux.results is detailed here. Compilation was also verified with clang.
The file name indicates the "payload": identity, payload, p1, p2... Refer to batch_optimization.cpp for details. Each file line has the following format:
<algorithm name (2 words)> <batch_size (1 integer)> <latency1 (integer, microseconds)> <latency2> ...
Hardware: Xeon-5 2620 v2 OS: linux 6.6 Compiler: gcc 4.8.3.4 Language: c++ 11 Compilation flags: -std=c++11 -DNDEBUG -O3 -funroll-all-loops Time unit: microseconds Hash function: fast hash 64 bits Memory alignment: valloc Data type size (int): 4 bytes Data size: M = 1073741824 bytes Number of data lookups per repetition: N = 1048576 Number of repetitions: n_reps = 100 Other: all runs with taskset -c 7 (pinned to CPU 7)
Architecture: x86_64 CPU op-mode(s): 32-bit, 64-bit Byte Order: Little Endian CPU(s): 12 On-line CPU(s) list: 0-11 Thread(s) per core: 2 Core(s) per socket: 6 Socket(s): 1 NUMA node(s): 1 Vendor ID: GenuineIntel CPU family: 6 Model: 62 Stepping: 4 CPU MHz: 1200.000 BogoMIPS: 4190.02 Virtualization: VT-x L1d cache: 32K L1i cache: 32K L2 cache: 256K L3 cache: 15360K NUMA node0 CPU(s): 0-11
